The present invention concerns a hydraulic system for a stepless transmission of a motor vehicle with a variator which possesses a first, drive side conical disk pair and a second power takeoff conical disk pair, between which an encircling member (e.g. an endless belt) runs, the first conical disk pair possesses one sliding, conical primary disk which can be controlled in an axial direction and the second power takeoff conical disk pair likewise possesses a sliding conical secondary disk which can be controlled in an axial movement and said primary and secondary disks are positioned by a hydraulic control apparatus, which apparatus has electromagnetic positioning members and hydraulic valves, the function of which apparatus is regulated by an electronic control system, and said apparatus has a cooler placed within a hydraulic oil lubricating/sump circulation for cooling the oil which leads from the cooler through a line to the sump.
Stepless automatic drives of this type, conventionally known as the CVT (Continuously Variable Transmission), possess, along with the above, an initiating device in the form of a hydro-dynamic converter, a forward/reverse unit, an intermediate shaft, and a differential. These are generally powered by the engine of the motor vehicle through the drive shaft. The forward/reverse unit comprises, in most cases, a planetary gear assembly with at least one sun gear, several planet gears, a hollow shaft, a brake and a wet multi-plate clutch.
The variator is comprised of two conical disk pairs and one encircling member whereby each conical disk pair is composed of one first conical disk which is fixed in its axle position and a second conical disk which is displaceably slidable along its axle. The controllable sliding conical disk, placed on the drive side, is designated as the primary disk and the controllable sliding conical disk, placed on the power takeoff side, is designated as the secondary disk. The encircling member which runs between the two conical disk pairs is usually a steel thrust belt. By means of the displacement of the slidable disk along its axle, the effective diameter for the belt is changed, and thereby the speed ratio of the stepless transmission changes accordingly. Meanwhile, the second conical disk pair is nonrotatably affixed to a power take off shaft which transmits the torque to an intermediate shaft, which governs the direction of rotation and matches the rate of rotation and torque by means of a toothed gear pairing. The torque of the intermediate shaft is then transferred to the differential by means of a further toothed gear pairing.
Control of the stepless automatic transmission is carried out by the electronic control apparatus by means of the hydraulic equipment, wherein the electronic control apparatus includes electromagnetic positioning members as well as hydraulic valves. A pump supplies oil from the oil sump to the hydraulic control equipment and to lubrication points of the transmission.
EP A 634 590 discloses a hydraulic system for the separation of the primary and/or secondary conical disks by means of a hydraulic valve. As this is done, the first valve controls the primary disk, and the second valve regulates the secondary disk. Both the first and the second valves are servo-controlled by an electromagnetic pressure controller. A pump supplies the second valve with a pressure medium, while the pressure supply to the first valve is adjusted by a control means for the second valve. The pressure level of the supply line to the first valve is, in this operation, identical to the pressure level delivered by the pump.
DE A 195 33 976 proposes a hydraulic system for the control of a stepless automatic transmission, this system possessing a pump delivering to a central first hydraulic branch, which branch supplies, for example, the primary disk, the secondary disk, the hydrodynamic converter, the lubrication points as well as the clutch. A second hydraulic branch is connected to the first hydraulic branch by means of a pressure reducing valve and attends to a series of electromagnetic pressure controllers.
During operation, in order to cool the self-heated oil of a stepless automatic transmission of this kind, a cooling circuit is installed in many transmissions, in which the hydraulic system is provided with a cooler through which the oil passes. This is especially the case when the stepless transmission is provided with an intake throttled radial piston pump for its oil supply, wherewith its flow of fluid is limited to a specified value. In this case, the quantity of oil which passes through the cooler is essentially dependent upon the volume of flow of the entire transmission, and particularly dependent on the oil requirement of the variator upon positioning the primary and secondary conical disks as well as dependent upon the internal leakage of the control system.
In transmissions of this type, experience has shown that at full capacity operation, the available quantity of oil which passes through the cooler and is limited by fixed orifices is not sufficient to remove completely the evolving heat therein. Wherein the transmission capability of said clutch must be sufficient for usual driving maneuvering, in spite of the volume requirements for displacement of the primary and secondary disks of the variator, the oil flowing through the cooler is further limited giving consideration which need to be given to the cooling requirements of the outflow of oil from the converter clutch.
Thus, the purpose of the invention is to propose a hydraulic system which, even with an automatic stepless transmission of the type wherein provision has been made for an intake throttled radial piston pump, a sufficient cooling of the oil for the heat transfer needs of the converter clutch is assured.
The invention is based on a situation wherein, during full capacity operation and at high speed of the vehicle, the variator can be driven at a single point in said operation, at the so-called TOP-gear ratio. This means that, at this point, no high-dynamic change in the variator can be brought about. As far as volume flow balance, this has as a sequential effect such that more oil flows through the cooler.
The present invention provides that an orifice be placed in the cooling circuit, particularly in a line connecting the cooler with the sump. The flow through the orifice is a set function based on either the rotational rate of the drive motor of the vehicle and/or on the velocity. This function is represented by values between which the orifice is controllable from a set point at which it is nearly closed to a second setting at which it is open. With this arrangement, the overload of oil can be delivered to the cooler and then sent to the oil sump independently of the rate of rotation and/or the velocity of the vehicle.
The installation of the rotational rate dependent orifice in the lubrication/cooling circuit is carried out after the cooler, as seen in the direction of oil flow, whereby this orifice is located on the low pressure side of the pressure system which serves the variator. If the orifice is controlled by the rotation rate of the engine of the vehicle, then this rotation must be representative of the speed of the vehicle. Further, it is advantageous to formulate the orifice so that the outflow through the orifice only begins after a specified driving velocity is attained. After that point, the further adjustment of the orifice will be based on the quantity of oil passing through as a function of the rate of rotation. The provision of pressure for the variator can, in the conventional manner, be taken care of by the main pressure valve which regulates the system pressure.
When this is done, the variator is connected to the high pressure side and the lubrication/cooling circulation is on the low pressure side of the main pressure valve.